The tennis superstar, 37, is mother to one-year-old Alexis Olympia, her first child with internet entrepreneur Alexis Ohanian.
On Wednesday, Williams posted a picture showing Alexis Olympia holding a tennis ball while wearing a ballet dress and a pink bow in her hair.
The image was captioned: “I will always protect you. I will always stand up for you. I will always guide you. I will always be with you.
“I was put here to be your mom. I love you like nothing I’ve ever loved before. I thank Jehovah everyday for you. Your momma.”
Last month Williams, the winner of 23 grand slam singles titles and widely considered one of the greatest tennis players of all time, revealed her family do not celebrate birthdays due to their religion.
During a press conference at the US Open, she said: “We’re Jehovah’s Witnesses so we don’t do that.”
Williams and Ohanian, who is the co-founder of Reddit, married in November 2017.